Write slides on interrupts

[jonathanpallant]

Perhaps a classroom analogy would work well here (some students put their hands down automatically, some students must be told to put their hands down).

Use the USBD power interrupt as an example, as it ties in well with the nRF52 USB Workbook.

[jonathanpallant]

Cover cs::Mutex<core::cell::RefCell<Option<THING>>> for sharing state with interrupt handlers.

[jonathanpallant]

We have example code for this now, but not slides.